1. Field of the Invention
The present invention pertains to medical implant devices, such as, without limitation, orthopedic implants, cardiac implants, dental implants, general surgical implants, neurological implants, gastrointestinal implants, urological implants, gynecological implants, or some other implantable medical device, and, in particular, to a method and apparatus for powering such a medical implant device that includes a wireless transponder and/or for reading such a medical implant device having an associated sensor.
2. Description of the Related Art
U.S. Pat. Nos. 7,333,013 and 7,932,825 describe a system wherein a medical implant device, such as, without limitation, an orthopedic (e.g., an artificial knee or hip) or cardiac implant (e.g., a pacemaker), has a radio frequency identification (RFID) tag mounted thereto. The RFID tag stores information relating to the implant and/or the patient, such as, without limitation, the implant type/model/serial number, the implant manufacturer, the procedure date, the hospital and/or the implanting surgeon. That information may later be obtained from outside of the patient's body when needed by reading the RFID tag using a properly equipped RFID reader device. For example, that information can be read by the surgeon or another healthcare professional during a post operative or later visit in order to obtain information needed by that individual during the visit in order to properly examine and treat the patient. As another example, that information can also be read and transmitted to a secure patient database for use in medical outcomes research performed by, for example, a healthcare organization or implant manufacturer.
In one particular implementation, described in U.S. Pat. Nos. 7,333,013 and 7,932,825, the RFID tag mounted to the implant is a passive RFID tag that includes an antenna, and may be read using a conventional RFID reader that is structured to read the implanted passive RFID tag over an air interface. In another particular implementation, an alternative RFID reader may be used, wherein the RFID reader is structured and configured to read the implanted RFID tag by making a direct (i.e., non-air interface) electrical connection to the RFID tag through the patient's living tissue using a probe provided as part of the RFID reader device (e.g., using transcutaneous contact and transcutaneous near field communication (TNFC) or transcutaneous for field communication (TFFC)). This latter implementation is described in U.S. Pat. Nos. 6,487,844, 7,228,183 and 7,825,807. In both of these implementations, the implanted RFID tag is powered by harvesting energy from the RF energy provided by the RFID reader.
Certain passive RFID tags provide a voltage output connection for powering other devices when such power is available from the energy harvested from an RFID reader. One use of this voltage is to power implanted sensors, which may be very simple in design because the RFID system provides a convenient method to communicate with the associated electronics.
As is known in the art, and as shown in FIG. 1, prior art RFID readers power and communicate with a passive RFID tags through relatively short bursts 2 of radio frequency (RF) energy (also known as RF pulses having a square wave format). The duration of these bursts 2 is limited by the FCC.
One type of sensor often used in association with passive RFID tags changes its conductivity when connected in an electrical circuit in order to report variations in the parameter to be measured. In one such embodiment, the sensor device is a voltage divider and is typically categorized as a resistor, i.e., a variable resistor. While an ideal variable resistor will essentially have an instantaneous response to an RF burst, such as burst 2, from an RFID reader, actual sensor implementations have dynamics associated with them where it takes a nonzero time for the sensor to reach a steady state output value. This behavior is similar to that of a resistive/capacitive (RC) circuit, exhibiting what is termed an RC time constant (shown in FIG. 2), or, alternatively, a resistive/inductive (RL) circuit, exhibiting what is termed an RL time constant (shown in FIG. 3), or some combination of both RC and RL dynamics.
If the RC and/or RL dynamics of the sensor are sufficiently fast, the reading can be accomplished within the time duration) of the energy burst of the RFID reader (e.g., within the duration, T, of burst 2 shown in FIG. 1). However, if the RC and/or RL dynamics of the sensor are such that the sensor is not able to reach an accurate steady state value within the duration of the energy burst of the RFID reader (e.g., within the duration, T, of burst 2 shown in FIG. 1), then problems will arise. In such a case, in order to get an accurate reading, the time during which RF energy used for powering is provided must be extended. However, FCC regulations limit the duration of RF bursts that may be output by an RFID reader or similar device over an air medium (i.e., the regulations set a maximum length for such duration). Thus, due to the FCC regulations, manufacturers of commercial off the shelf (COTS) RFID reader devices will not be willing to alter their devices to allow them to exceed this regulated limit because COTS RFID reader devices are specifically designed to transmit over an air medium. Furthermore, while the application of the reader in the transcutaneous method described above is not an over the air regulated situation, asking the COTS reader manufacturers to alter their reader would defeat the COTS availability of such readers for a much wider market.